1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and is suitable for use in, for example, a video camera, an electronic still camera, a TV camera (broadcasting camera), and a silver-halide film camera.
2. Description of the Related Art
In recent years, an image pickup apparatus such as a video camera, a digital still camera, and a broadcasting camera which use a solid-state image pickup element, or a camera using a silver-halide film is improved in function, and the entire image pickup apparatus is reduced in size. A zoom lens which has a short total lens length, is compact, and has high resolution is desired as an optical system used for the image pickup apparatus.
In order to reduce a size of the zoom lens, the number of lenses is desirably reduced while a refractive power of each of lens units included in the zoom lens is increased. However, a lens thickness increases along with the increase in refractive power of each surface, with the result that the length of the entire lens system cannot be sufficiently reduced and it makes it difficult to correct various aberrations.
When the respective lens units are to be collapsed in a case where a camera is not used, errors such as the tilt of lenses or lens units become inevitably larger because of mechanical structure. In such case, when the sensitivity of lenses or lens units is excessively large, the degradation of optical performance or image fluctuation during zooming occurs. Therefore, it is desirable to minimize the sensitivity of lenses or lens units in the zoom lens.
In a rear focus zoom lens in which lens units other than a first lens unit located on the object side are moved for focusing, an effective diameter of the first lens unit can be reduced compared with the case where the first lens unit is moved for focusing, and hence the entire lens system is easily reduced in size. The lens units which are small in size and light in weight are moved for focusing, and hence the driving power of the lens units is small, with the result that focusing can be speedily performed.
There has been known, as the rear focus zoom lens, a zoom lens which is suitable for a retractable structure and includes four lens units, that is, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, which are provided in order from the object side to the image side.
U.S. Pat. No. 6,016,228 discloses a zoom lens having a zoom ratio of approximately 3. The first lens unit is constituted by a single lens. Intervals between the respective lens units are adjusted for zooming. The fourth lens unit is moved for focusing. The zoom lens is suitable for a retractable structure because the sensitivity of the first lens unit or the second lens unit is relatively small. However, the first lens unit is not moved during zooming, and hence it is difficult to shorten a total lens length at the wide-angle end and reduce the diameter of a front lens.
U.S. Pat. No. 6,975,461 discloses a zoom lens including respective lens units to be moved for zooming and having a zoom ratio of approximately 5. In the zoom lens, a share of the third lens unit in magnification is suitably set, whereby the zoom lens having a zoom ratio of approximately 5 is realized. In order to further increase the magnification of the zoom lens, it is necessary to suitably set a share in magnification between the second lens unit and the third lens unit.
U.S. Pat. No. 7,206,137 and U.S. Pat. No. 7,190,529 disclose a zoom lens including respective lens units to be moved for zooming and having a zoom ratio equal to or larger than 10.
In the zoom lens disclosed in U.S. Pat. No. 7,206,137, a focal length of the first lens unit is suitably set to achieve a balance between a reduction in size of the entire system and correction of various aberrations. However, a moving distance of the first lens unit during zooming is large, and hence a multi-step lens barrel structure is required in order to shorten a length in a collapsed state.
In a zoom lens disclosed in U.S. Pat. No. 7,190,529, a change in lateral magnification of the second lens unit during zooming is equal to or smaller than 15, and hence it is difficult to increase the magnification.
In the lens constitution disclosed in each of U.S. Pat. Nos. 7,206,137 and 7,190,529, in order to further increase the magnification, it is necessary to increase a share of the fourth lens unit in magnification. However, when the imaging magnification of the fourth lens unit is increased at the telephoto end, focal moving distance when the fourth lens unit is moved in the optical axis direction becomes smaller. Therefore, when a focal variation caused by a change in temperature or a tolerance in parts is to be corrected using the fourth lens unit, the amount of driving becomes larger. This is not desirable.
On the other hand, when the imaging magnification of the fourth lens unit is reduced, the share of the fourth lens unit in magnification becomes smaller, and the refractive powers of the other lens units and the moving distance thereof during zooming necessarily increase. Therefore, it is difficult to ensure high optical performance while a compact lens barrel having the retractable structure is provided.
There has also been known a zoom lens in which lens unit(s) constituting the zoom lens is (are) displaced in a direction perpendicular to the optical axis to correct a blurred image caused when the zoom lens vibrates (U.S. Pat. No. 5,585,966). The zoom lens includes four lens units, that is, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, which are provided in order from the object side to the image side. The entire third lens unit is vibrated in the direction perpendicular to the optical axis to correct the blurred image.
In the zoom lens including the four lens units, in order to obtain excellent optical performance while a high zoom ratio is realized and the entire lens system is reduced in size, it is important to suitably set refractive indices and lens constitution of the respective lens units and movement conditions of the respective lens units in the case of zooming.
In particular, it is important to suitably set the movement conditions of the respective lens units in the case of zooming, the refractive power of the second lens unit (reciprocal of focal length), and the imaging magnification of the fourth lens unit.
When such configurations are not suitably set, it is very difficult to obtain a zoom lens which has high optical performance over the entire zoom range and is suitable for the retractable structure, while a high zoom ratio is ensured.